The present invention concerns water-redispersible polymer powders, particularly water-redispersible powders wherein the polymer comprises in copolymerized form carboxylated copolymers of vinyl aromatic comonomers and 1,3-diene comonomers.
Water-redispersible powders made from the above-mentioned polymers and their use in the construction industry are generally known. Carboxylated vinyl aromatic-1,3-diene copolymers have many advantages, such as high flexibility, high adhesion to polar substrates and high adaptility of the copolymer to the particular end-use by varying the ratio between the “hard” and “soft” comonomers, i.e. the vinyl aromatic comonomer and 1,3-diene comonomer.
U.S. Pat. No. 6,632,861 discloses self-leveling screeds and trowel-applied flooring compositions which contain a dispersion powder composition based on vinylaromatic-1,3-diene copolymers.
The polymer powders are typically produced by subjecting an aqueous dispersion of the polymer to a drying operation in which its volatile components are evaporated, for example by means of spray drying or freeze drying. The evaporation of the aqueous dispersion medium may be accompanied by irreversible aggregation of the polymer particles of the aqueous dispersion with one another, to form secondary particles. The formation of secondary particles results in poorer redispersibility, which is generally accompanied by poorer performance properties of the powder. Therefore, good redispersibility in water is one of the most important properties of the water-redispersible polymer powders. A large variety of spray-drying aids are used in the known processes of producing polymer powders to improve their water redispersibility.
WO 97/38042 relates to redispersible powders based the above-mentioned polymers. To achieve a good redispersibility of the polymer in water, spray-drying aids are added to the polymer suspension before spray-drying. The spray-drying aids are a blend of a salt of alkylated di-phenyl-ether-di-sulphonic acid, caseinate and/or N-alkysulfosuccinamide, and/or a di- or tetra-sodium or potassium salt of an addition product N-alkyl-sulphosuccinamate with maleic acid mixed with polyvinylalcohol.
European Patent No. 1,065,226 relates to a process of preparing redispersible powders based on the above-mentioned polymers wherein a hydroxycarboxylic acid or a salt thereof is added to an aqueous dispersion of an above-mentioned copolymer in an amount of 3-40 wt.-% of a hydroxycarboxylic acid or a salt thereof, based on the amount of the polymer, and the dispersion is dried.
U.S. Patent Application Publication No. 2007/0004834 discusses known spray-drying aids in the production of water-redispersible polymer powders as well as their disadvantages, particularly the formation of fissured films produced from these polymers or the strong smell of the produced polymer powders. The '834 Publication discloses the use of polyvinylformamide as a spray-drying aid to produce a polymer powder which is without odor or color, and exhibits effective redispersibility and filming.
In view of the wide variety of end-uses of water-redispersible polymers, evidently one single production method or one single spray-drying aid cannot satisfy all needs of the industry. Accordingly, one object of the present invention is to provide new water-redispersible polymer powders which comprise carboxylated copolymers of vinyl aromatic comonomers and 1,3-diene comonomers and processes for producing them. A preferred object of the present invention is to provide such water-redispersible polymer powders which exhibit a good water redispersibility. Another preferred object of the present invention is to provide such water-redispersible polymer powders which do not have a strong odor.